This invention relates to the selective removal of H.sub.2 S from gas mixtures which contain both H.sub.2 S and CO.sub.2. More particularly the invention is concerned with the selective removal of H.sub.2 S from such mixtures in which the CO.sub.2 to H.sub.2 S molar ratio is relatively high, generally 4 CO.sub.2 :1 H.sub.2 S or greater, and in which the gas mixtures to be treated undergoes substantial variations in flow rate.
In a number of industrial applications gas mixtures containing relatively small amounts of H.sub.2 S and relatively large amounts of CO.sub.2 are encountered, and in the treatment of these gases it is often desired to selectively remove the H.sub.2 S component, that is, to maximize H.sub.2 S removal while minimizing CO.sub.2 removal. One such application, with which this invention is particularly concerned, involves the treatment of gases produced by the gasification of sulfur-containing oil, coal or other carbonaceous materials. For example in the gasification of sulfur containing coal with steam and air to produce a so-called low BTU gas, the gas produced may typically contain from 0.5% to 2% H.sub.2 S and 8% to 16% CO.sub.2 (all percentages on a molar basis). When such gas mixtures are to be used in power generation cycles, it is usually highly desirable to provide for selective H.sub.2 S removal, reducing the H.sub.2 S residual to as low a level as possible, while at the same time removing as little CO.sub.2 as possible. The removal of H.sub.2 S to low residual levels is usually required for environmental reasons, that is to minimize the discharge of sulfur oxides to the atmosphere which would occur if the H.sub.2 S were permitted to enter the combustion portion of the power cycle. The removal of the sulfur content of the gas as H.sub.2 S is generally much less costly and more energy efficient than its removal as sulfur oxides from the stack gases. Minimizing CO.sub.2 removal is desired for a number of reasons. First, in a power generation cycle removal of CO.sub.2 reduces the gas volume and thereby reduces the energy efficiency of the cycle. Second the removal of the CO.sub.2 requires the expenditure of energy and thereby further reduces energy efficiency. Thirdly, the removal of large amounts of CO.sub.2 complicates the treatment of the CO.sub.2 /H.sub.2 S mixtures removed from the gas. Normally such CO.sub.2 /H.sub.2 S mixtures are treated by the so-called Claus process to oxidize the H.sub.2 S to elemental sulfur. The operation of the Claus unit becomes inefficient when the H.sub.2 S content of the CO.sub.2 /H.sub.2 S mixture drops below about 15%. By selectively removing H.sub.2 S from a gas having a high CO.sub.2 : H.sub.2 S ratio, e.g. 15:1, the CO.sub.2 /H.sub.2 S mixture recovered may for example be enriched in H.sub.2 S from about 6% H.sub.2 S if non selective removal is used to a level of e.g. 15% to 20% H.sub.2 S, at which higher level it can be treated in a Claus plant with reasonable efficiency.
It has been known for some time that selective removal of H.sub.2 S from gas mixtures containing both CO.sub.2 and H.sub.2 S may be accomplished by scrubbing the gas mixture with aqueous potassium carbonate solutions. See for example U.S. Bureau of Mines Report of Investigations 5660 "Removing Hydrogen Sulfide by Hot Potassium Carbonate Absorption" by J. H. Field et al, U.S. Pat. No. 3,931,389 also discloses the use of potassium carbonate solutions to selectively absorb H.sub.2 S from gases containing both CO.sub.2 and H.sub.2 S.
Prior systems of this type employing potassium carbonate solutions, have been mainly based on the marked difference in the absorption rate of H.sub.2 S in potassium carbonate solutions as compared to CO.sub.2. Generally speaking, the absorption rate of H.sub.2 S in aqueous potassium carbonate is about ten times or higher than the rate of CO.sub.2 absorption. Advantage is taken of this difference in absorption kinetics by designing the absorption column with relatively low mass transfer capacity, thus limiting the amount of CO.sub.2 absorbed by limiting the time of contact with the absorbent solution.
While good H.sub.2 S selectively can be obtained when the volume of gas to be treated remains constant by taking advantage of the differential in absorption kinetics between H.sub.2 S and CO.sub.2, it has been found that this approach is not satisfactory in systems in which the gas volume to be treated undergoes substantial variation. In such systems, the H.sub.2 S selectivity achieved undergoes wide variations as the gas flow varies. With the system designed to achieve the desired degree of purification and the desired degree of H.sub.2 S selectivity at the maximum gas flow rate, the selectivity will fall off sharply as the gas flow rate falls off. This is due to the increased residence time for absorption as the gas flow decreases with a constant mass transfer volume in the absorption column. Because of the sharp differences in the gas-liquid equilibrium values for CO.sub.2 and H.sub.2 S, the driving forces for CO.sub.2 absorption (i.e. the difference between the partial pressure of CO.sub.2 in the gas phase and the equilibrium partial pressure of CO.sub.2 above the scrubbing solution) are far greater for CO.sub.2 than the corresponding driving forces for H.sub.2 S absorption. As a consequence, the increased residence time will greatly increase CO.sub.2 absorption while only slightly or negligibly increasing H.sub.2 S absorption. The large relative increase in CO.sub.2 absorption at lower gas flow rates sharply reduces the desired degree of selective H.sub.2 S absorption. While it is theoretically possible to vary the mass transfer capacity of the absorber during operation (for example by using multiple absorbent solution inlets at different levels in the absorber) in practice such systems are expensive and difficult to design and operate, particularly when the gas flow changes rather rapidly and at frequent intervals.
Power generation systems utilizing gases which require selective H.sub.2 S removal are good examples of systems involving relatively large variations in gas flow to the purification system in response to varying load demands on the power generation system. Often such systems involve a four-fold variation in gas flow to the purifier with the changes in flow often taking place in a period of minutes. In a purification system based on absorption kinetics the selectivity of H.sub.2 S removal falls off rapidly as the gas flow decreases with the result that at decreasing loads greater amounts of CO.sub.2 are absorbed and the off-gases from the regenerator become leaner and leaner in H.sub.2 S.